This disclosure relates to high flow polyphenylene ether formulations with improved properties.
Polyphenylene ether resins (PPE) are an extremely useful class of high performance engineering thermoplastics by reason of their hydrolytic stability, high dimensional stability, toughness, heat resistance and dielectric properties. They also exhibit high glass transition temperature values, typically in the range of 150° to 210° C., and good mechanical performance. This unique combination of properties renders polyphenylene ether based formulations suitable for a broad range of applications which are well known in the art. One example is injection molded products which are used for high heat applications. Polyphenylene ether polymers typically have relatively high molecular weights and possess high melt viscosity with intrinsic viscosity values typically greater than about 0.3 dl/g, as measured in chloroform at 25° C.
One area in which polyphenylene ether based compositions have required an improvement is melt flow capability, i.e. the ability to flow freely at elevated temperatures during various processing stages such as extrusion and molding. Poor melt flow can impact the size and type of part which can be prepared with the composition. In U.S. Pat. No. 4,154,712 to G. Lee, Jr. teaches that processability can be improved by decreasing the molecular weight of the polyphenylene ether polymers; however, lower molecular weight sometimes adversely affects other properties such as impact strength. To aid processing, polyphenylene ether resins are often prepared with flow promoters such as polystyrene, high impact polystyrene, saturated polyalicyclic resins and terpene phenol to impart high flow to the resulting composition. Polystyrene, terpene phenol and other such flow promoters reduce the heat deflection temperature (HDT) of the product and typically increase the flammability of the PPE resin, as measured under UL94 standard protocol.
Polyphenylene ether resins are often flame retarded with phosphorous containing organic compounds such as resorcinol diphosphate, Bisphenol-A diphosphate and tetraxylyl piperazine diphosphoramide to comply with Eco-label guidelines. The fire retardant grades of polyphenylene ether, particularly those rated UL94 V0, tend to be formulated using large amounts of flame retardant additives. The addition of large amounts of these phosphorous containing organic compounds reduces the heat deflection temperature (HDT) of the formulation even further.
Efforts to improve the flow characteristics of PPE resins with minimal or no loss of HDT values and impact properties have been made. For example, U.S. Pat. No. 5,376,724 to Bailey et al. discloses polyphenylene ether compositions which contain a resinous additive that improves flow with only minor reductions in HDT values and impact strength. The resinous additive is said to be comprised of vinyl aromatic monomers such as styrene monomers or a hydrocarbon compound containing at least 35 wt % aromatic units.
In addition, U.S. Pat. No. 5,081,185 to Haaf et al. describes compositions comprising a blend of two or more polyphenylene ether resins with one resin having high intrinsic viscosity values of at least about 0.38 dl/g and the other having low intrinsic viscosity values of no greater than 0.33 dl/g. The blend of the two PPE resins exhibits higher melt flow with no substantial decrease in heat deflection temperature (HDT) when compared to the high intrinsic viscosity PPE resin of the blend. Haaf et al. provides no special means for isolating low I.V. PPE resins from a reaction solution. In conventional isolation techniques, the PPE reaction solution, typically in toluene, is added to a non-solvent, such as methanol, to precipitate the PPE resin product. These non-solvent precipitation techniques provide very fine particles when applied to low I.V. PPE resins. These fine particles have very low bulk densities and are difficult to feed into extruders and other processing equipment. Consequently, the compositions taught by Haaf have limited commercial utility due to poor compounding throughputs and often erratic operation. Fine particles are also formed when isolating ultra-low viscosity PPE resins, e.g., PPE having an I.V. of less than about 0.27 dl/gm as measured in chloroform at 25° C., from solution by precipitation with a non-solvent.
It is desirable to provide a high flow PPE resin composition with improved flame retardance as well as improved HDT values and impact properties. Additionally it is desirable to develop improved methods to manufacture such compositions.